Cable TV and other types of electronic mass audience distribution systems such as broadcasting by domestic wireless television stations or via satellite have distributed more than only television signals in recent years. For example, such systems transmit closed caption text for the hard of hearing, that can be decoded by a closed caption decoder, they transmit descrambling codes to be received by pay TV customers of a cable TV system operator, videotext is transmitted and displayed on a TV screen as text or simple graphics files, they download games from a head end which can be played via a TV set, etc. In more recent years, various other services are contemplated or have begun, such as banking, narrowcasting, electronic mail, facsimile broadcast or narrowcasting to individual subscriber stations, etc., all carried via an electronic mass distribution system.
The data which is transmitted typically has several characteristics in common. The data carries a header which contains a fixed or variable address of a particular recipient. In case the recipient is one of a specialized audience group, the header contains a common address of the group. The data is typically transmitted either during the 21st scanning line of a television channel, from where it is decoded by a decoder at the subscriber location, or it is sent in a separate data channel, and is decoded from the data channel at the subscriber location. One system of this kind that broadcasts data is described in U.S. Pat. No. 4,623,920, dated Nov. 18th, 1986, invented by Michel Dufresne et al, and entitled "Cable Network Data Transmission System".
Data to be transmitted is assembled at a head end in a RAMDISK, which is cyclically read in a sequential manner. The data that is read is interspersed with null data, in order to give receiver processors, which are considerably less expensive than the head end processor and thus are much slower in operation, time to process the received data. Null data is also inserted in order to allow a head end processor to insert specialized data into the data stream, such as electronic mail, which is to be transmitted only once to a single subscriber's terminal, in contrast to other data such as program announcements that is transmitted cyclically.
A block diagram of a system of the kind described above is shown in FIG. 1. An information supplier provides data signals which are stored on a hard disk drive of a minicomputer 1 such as a type PDP 11 at a head end. In addition a service provider, using e.g. an IBM compatible personal computer 3 provides data via a modem 5 and telephone line to a front end processor 7. Both the processor 7 and computer 1 are connected in a network 9, such as Ethernet.TM., to which a main processor 11 is connected. Another personal computer 12 is connected to the network, and is also connected to a RAMDISK 13. RAMDISK 13 is also connected to processor 11.
Computer 12 reads data from computer 1, and from processor 7 and writes it to the RAMDISK 13. Main processor 11 reads the data cyclically and sequentially from RAMDISK 13, and outputs it to digital to analog converter 15, which converts the data to analog form and presents it to modulator 17 for merging with video signals and subsequent transmission to a network 19 to which audience terminals have access.
It is possible for processor 7 to interrupt processor 11 and to cause processor 7 to insert data into the data stream. However, this data can only be inserted as it arrives on the network 9 (i.e. the data can be inserted once), and after the interrupt the processor 11 returns to the cyclic reading of the RAMDISK 13.
With the expected substantial increase in traffic to provide the aforenoted new services, the system shown in FIG. 1 has exhibited substantial problems. A first problem is a limitation in capacity. In order to accommodate an increase in the amount of data to be sent in one cycle, the capacity of the RAMDISK mass storage device must become larger and larger. This increases the read cycle time of the RAMDISK, and slows down to an unacceptable degree the responsiveness of the system to, for example, commands sent upstream requesting a service, such as an update report on a bank balance, (for example, delays of 20 seconds to provide a response have been experienced), or the updating of closed captioning (which could unacceptably lag the accompanied story being shown on television).
Secondly, the cyclic operation requires the insertion of null data on the RAMDISK in order to accommodate the aforenoted insertion of data during the null data. The use of null data imposes an inherent limitation on the capacity of the system. No more insert data can be accommodated than the capacity of the null portions of the signals, and the more null data stored on the RAMDISK, the less cyclic data can be transmitted.
Thirdly, the above-described system cannot prioritize data signals that are to be sent on a cyclic basis. For example, if an emergency signal is to be broadcast on a cyclic schedule, it must be written to the RAMDISK. This takes time. If the reading of the RAMDISK has just passed the memory location where the emergency signal has just been written, it could take the reading time of the entire RAMDISK (e.g. 20 seconds for a large system) before the emergency signal is read and transmitted. This could result in tragic consequences if the emergency is, for example, a missile alert, warning of a tidal wave or earthquake, etc.
Further, the above-described system cannot accommodate different data signals that are to be transmitted synchronously at different frequencies.